Laboratory Workup for Suspected Septic Joint
For suspected septic arthritis, obtain ESR and CRP (combined sensitivity/specificity is optimal), perform arthrocentesis with synovial fluid cell count with differential, Gram stain, and aerobic/anaerobic cultures, and obtain blood cultures if the patient is febrile or systemically ill. 1, 2
Essential Initial Serum Tests
Inflammatory Markers (Highest Priority)
- ESR and CRP should both be obtained in all patients when the diagnosis is not clinically evident, as the combination provides the best sensitivity and specificity 1, 2
- ESR ≥15 mm/h has 94% sensitivity for septic arthritis 3
- CRP ≥20 mg/L has 92% sensitivity for septic arthritis 3
- The combination of abnormal ESR and CRP together maximizes diagnostic accuracy 1
Complete Blood Count
- CBC with differential to evaluate for leukocytosis and systemic infection 2
- Elevated serum neutrophil percentage correlates with concurrent systemic sepsis 4
Blood Cultures
- Obtain blood cultures for aerobic and anaerobic organisms if fever is present, acute symptom onset, or suspected bacteremia (e.g., Staphylococcus aureus) 1
- Blood cultures are particularly important as concurrent bacteremia indicates higher risk of poor outcomes 4
Synovial Fluid Analysis (Critical for Diagnosis)
When to Perform Arthrocentesis
- Diagnostic arthrocentesis should be performed in all patients with suspected acute septic arthritis unless the diagnosis is clinically evident AND surgery is already planned AND antimicrobials can be safely withheld 1, 2
- Image-guided aspiration (fluoroscopy, ultrasound, or CT) should be used when needed to confirm intra-articular needle positioning 1
Required Synovial Fluid Tests
- Note: Gram stain should NOT be used as a "rule out" test due to limited sensitivity 1
Aerobic and anaerobic cultures 1, 2
- Positive synovial fluid culture is the reference standard for diagnosis 1
Crystal analysis if clinically indicated 1, 2
- Important to exclude crystal arthropathy, which can coexist with infection 2
Timing Considerations for Antimicrobial Therapy
Critical Pitfall to Avoid
- If the patient is medically stable, withhold antimicrobial therapy for at least 2 weeks prior to collecting synovial fluid for culture to increase organism recovery 1
- Do NOT start antibiotics before obtaining cultures when clinically feasible, as this significantly reduces diagnostic yield 2
- This recommendation does NOT apply to life-threatening septic episodes where immediate antibiotics are required 1
Additional Laboratory Tests Based on Clinical Context
Baseline Assessments Before Antibiotic Therapy
When Diagnosis is Uncertain
- Antinuclear antibodies, rheumatoid factor, and anti-CCP antibodies if autoimmune disease is suspected 2
Advanced Biomarkers (Prosthetic Joint Infections)
- Interleukin-6 2
- Synovial fluid alpha-defensin (provides high accuracy for prosthetic joint infections) 2
Prosthetic Joint Infections: Special Considerations
For suspected prosthetic joint infection (PJI), the same laboratory workup applies with these additions:
- ESR and CRP combination remains the best screening test 1
- Arthrocentesis with synovial fluid analysis (cell count, differential, cultures) is essential 1
- At least 3 and optimally 5-6 periprosthetic tissue samples should be obtained intraoperatively for culture 1
- Blood cultures if fever, acute onset, or suspected S. aureus infection 1
Common Clinical Pitfalls
- Do not delay arthrocentesis—synovial fluid analysis is required to confirm diagnosis and cannot be replaced by serum markers alone 2
- No single clinical sign or laboratory test (excluding bacteriological culture) is conclusive for differentiating septic from non-septic arthritis 6
- ESR alone has poor specificity (only 11%) and should not be used in isolation 7
- Chills, local redness, and entry site for infection are clinical features that increase likelihood of septic arthritis 6